This invention relates to fuel cells and, in particular, to a bipolar separator plate and anode side hardware design for use in molten carbonate fuel cells.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte, which serves to conduct electrically charged ions. Molten carbonate fuel cells (“MCFCs”) operate by passing a reactant fuel gas through the anode, while oxidizing gas is passed through the cathode. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate between each cell.
In internally reforming fuel cells, a steam reforming catalyst is placed within the fuel cell stack to allow direct use of hydrocarbon fuels such as methane, coal gas, etc. without the need for expensive and complex reforming equipment. Two different types of internal reforming have been used. Direct internal reforming is accomplished by placing the reforming catalyst within the active anode compartment of each fuel cell. The reforming catalyst in direct internal reforming fuel cells is typically placed in an anode current collector and is available to reform fuel gas with steam formed by the electrochemical reactions of the fuel cell and can result in very high reforming efficiency and fuel utilization.
However, direct internal reforming fuel cells experience a decay in the catalytic activity of the reforming catalyst. More particularly, over the operating life of the fuel cell, the molten carbonate electrolyte stored in the anode electrode of the cell, wets the abutting anode current collector to expose and eventually poison the reforming catalyst stored in the current collector. When the reforming catalyst is poisoned by the electrolyte, it is no longer able to perform the reforming reaction with the hydrocarbon fuels to generate sufficient hydrogen fuel for the anode reaction. The poisoning of the reforming catalyst thus reduces the reforming and electrical efficiencies of the fuel cell.
To reduce electrolyte wetting of the anode current collector, conventional systems have employed a corrugated anode current collector which can act as a barrier to shield the reforming catalyst from the electrolyte. A non-wettable barrier between the anode and the anode current collector, such as an anode support member, has also been used to impede the wetting of the anode current collector and the creepage of the electrolyte toward the reforming catalyst. For example, U.S. Pat. No. 5,558,948 discloses a support member in a form of a perforated plate member made from a metallic corrosion resistant material. Other conventional systems have employed anode support members in the form of an expanded mesh, a wire mesh, or a porous sintered powder bed (U.S. Pat. No. 6,719,946). An anode support member constructed of a high porosity reticulated foam material has also been disclosed (U.S. Pat. No. 6,379,883). Non-wettable materials used to form such anode support members typically include nickel, copper or other materials which are stable in the fuel-reducing atmosphere.
To further assist in retarding the creepage of the electrolyte, conventional systems have also employed a bipolar separator plate with a protective coating on the plate or on portions of the plate forming wet seal regions. Such protective coating is typically formed from Al or Al/Fe (JP Patent Application No. 09-025822), and is applied to the wet seal portions of the plate by thermal spraying (JP Patent Application Nos. 09-025822 and 07-295276), high velocity oxy-fuel flame spraying (U.S. Pat. No. 5,698,337), aluminum painting, ion vapor deposition or molten aluminum dip-coating (JP Patent Application No. 07-230175). For example, U.S. Pat. No. 6,372,374 discloses a bipolar separator plate design which uses a stainless steel center sheet and wet-seal pocket members fabricated from stainless steel with aluminum protective coating. In the '374 patent, the wet-seal pocket members are welded to the center sheet and aluminum is included in the weld material in a form of Al-containing filler wires. The separator plate and the anode current collector can also be coated with nickel or copper by electrolytic plating, cladding or vacuum deposition so as to further retard the electrolyte creepage to the reforming catalyst. For example, U.S. Pat. No. 5,698,337 discloses a bipolar separator plate comprising Ni-clad stainless steel.
While the above methods have been successful in slowing down the rate of electrolyte creepage, these methods suffer from a number of disadvantages. The conventional anode support members, when used to form a barrier between the corrugated current collector and the anode electrode, have been unable to provide sufficient mechanical support for the anode electrode. This results in waviness of the anode electrode causing insufficient electrical contact between the anode electrode and the abutting electrolyte matrix. The aluminum coating on the wet-seal pocket members of the separator plate is oxidized during the fuel cell operation and the surface of the wet-seal pocket members of the plate becomes wettable by the electrolyte. This allows the electrolyte to creep along the external surface of the pocket members and toward the anode current collector and the reforming catalyst through fuel inlet and outlet edges of the separator plate.
Moreover, the coating or plating processes to coat the wet-seal pocket members of the separator plate or the anode current collector with nickel or copper are expensive and significantly increase the fuel cell system manufacturing costs. The conventional electrolytic plating process used to coat the wet seal pocket members or the current collector can also result in non-uniform thickness of the coated portions of the wet seal pocket members. Such non-uniformity in thickness causes flow and pressure mal-distribution within the fuel cell system as well as insufficient electrical contact between the fuel cell components.
It is therefore an object of the present invention to provide a fuel cell assembly including an anode support, an anode current collector and a separator plate which overcomes these disadvantages.
It is also an object of the present invention to provide an improved anode support member which further reduces the electrolyte wicking rate to the reforming catalyst so as to prolong the catalyst life and provides improved mechanical support for the anode.